Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already declare readOnly, idempotent, non-destructive, non-openWorld, so the safety profile is covered. The description adds genuinely new context: the voice taxonomy (2 Japanese standard voices plus age-tagged Gemini-TTS voices) and the pricing note that cost does not vary by voice – useful traits not in the annotations.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

Three tight sentences, front-loaded with what the tool returns, then where the values are used, then catalog/pricing facts. No filler; the api_key note is parenthetical.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a single-parameter read-only lister with no output schema, the description covers purpose, consumers, catalog composition, and cost behavior. It omits what fields each voice record returns, but with no output schema that is a minor gap.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage is 100% and the single parameter (api_key) is fully documented in the schema. The description mentions api_key needs to be issued from the admin console, adding minor operational context, but nothing about the parameter's syntax beyond what the schema provides. Baseline 3.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

States a specific verb+resource ('使える声 (voice_id) の一覧' – list of available voices) and immediately distinguishes itself from siblings by naming the exact downstream consumers (create_session, create_avatar) that need the voice_id. An agent can route to this tool without opening any schema.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

Gives clear usage context by naming the two sibling tools that consume voice_id, which tells the agent when to call this first. It does not give explicit when-not-to-use or an alternative lister, but for a leaf enumeration tool this is strong.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.